The present invention relates to a variable focal length lens used as a picture-taking lens of a camera. Concerning a variable focal length lens such as a conventional zoom lens and also concerning a single focal length lens, the following focusing systems are adopted.
The first focusing system is described below. In the first focusing system, only a specific lens component is moved so that focusing operation can be conducted from the infinite to a short distance. This example is shown in FIG. 1. When zooming is conducted in a range from W (short focal length) to T (long focal length), both the front component lens FC and the rear component lens RC are moved. In this case, focusing is conducted at a predetermined focal length in such a manner that only the front component lens FC is protruded in the arrowed direction in FIG. 1. In this connection, reference character F represents a photographic film.
According to the above system, a distance of the movement of the lens component is short. As a result, dimensions of the mechanism to move the lens barrel are small. Due to the above advantages, this system is most commonly used for the zoom lens at present.
The second focusing system is described below. In the second focusing system, the entire lens components are moved so that focusing can be conducted in a range from the infinite to a short distance. This system is shown in FIG. 2. In this system, in the case of focusing, both the front component lens FC and the rear component lens RC are protruded while leaving a predetermined interval between them. Due to the foregoing structure, this system is advantageous in that the lens performance is not deteriorated in the process of focusing, that is, the occurrence of aberration can be avoided.
In this system, the lens components are protruded while leaving a predetermined interval between them. Accordingly, the lens performance is not deteriorated in the process of focusing. Therefore, this system is mainly used for a single focal length lens.
However, the following problems may be encountered in these two conventional systems.
In order to downsize a camera, it is necessary to reduce the overall length of the lens components. Accordingly, in the first system, when the zoom lens is downsized, it is necessary to conduct focusing in accordance with a change in the object distance, and it is required to move a specific lens component by a very short distance. As a matter of fact, when a distance of the movement of the lens component is very small, it is difficult to adjust a position of the lens component with high accuracy. In general, the focal depth of a zoom lens is small on the telescopic side. Accordingly, the number of positions at which the specific lens component must be stopped for focusing is increased. For the reasons described above, it is difficult to downsize the lens unit. On the other hand, as compared with the second system described above, the lens performance is deteriorated on the wide angle side of a zoom lens in the first system, that is, aberration tends to occur on the wide angle side of a zoom lens.
In the second system, the following problems may be encountered. The longer the focal length is increased, the longer the distance of movement of the lens component for focusing is increased. Further, when focusing is conducted, the focal length is greatly changed.
In any variable focal length lenses of the prior art, it is difficult to realize a mechanism with high accuracy by which lens components A and B are integrally moved in the optical axis direction in one region, and in the other region, lens components A and B are respectively independently moved.
For example, even if the cams to move the lens components A and B are respectively set up so that the above conditions can be satisfied and the cams have the same inclination in a specific region, it is impossible to move the lens components A and B completely integrally with each other. Therefore, it is difficult to ensure the accuracy. In the case where a helicoid is used, it is impossible to realize the above mechanism by this method.
In the variable focal length lens of the prior art, it is impossible to realize the following mechanism. That is, when one lens component B is moved in the direction of the optical axis, the other lens component A is released from the drive means of its own such as a cam and helicoid, so that the lens component A can be moved together with the lens component B.
The present invention has been accomplished to solve the above technical problems caused when the variable focal length lens is downsized.